Chapter 13 Meiosis and Sexual Life Cycles. Overview: Hereditary Similarity and Variation Living...
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Transcript of Chapter 13 Meiosis and Sexual Life Cycles. Overview: Hereditary Similarity and Variation Living...
Chapter 13
Meiosis and Sexual Life Cycles
Overview: Hereditary Similarity and Variation
Living organisms Are distinguished by their ability to
reproduce their own kind Heredity
Is the transmission of traits from one generation to the next
Variation Shows that offspring differ somewhat in
appearance from parents and siblings
Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes
Genetics Is the scientific study of heredity and
hereditary variation Genes
Are the units of heredity Are segments of DNA
Genes
• Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs)
• Each gene has a specific location called a locus on a certain chromosome
• Most DNA is packaged into chromosomes
Comparison of asexual and sexual reproduction
In asexual reproduction One parent produces genetically identical
offspring by mitosis (clones) In sexual reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Fig. 13-2
(a) Hydra (b) Redwoods
Parent
Bud
0.5 mm
Concept 13.2: Fertilization and meiosis alternate in sexual life cycles
A life cycle Is the generation-to-generation sequence of
stages in the reproductive history of an organism
In humans Each somatic cell has 46 chromosomes, made
up of two sets One set of chromosomes comes from each
parent
5 µmPair of homologouschromosomes
Centromere
Sistechromatids
Figure 13.3
A karyotype
Is an ordered, visual representation of the chromosomes in a cell
Fig. 13-3b
TECHNIQUE
Pair of homologousreplicated chromosomes
Centromere
Sisterchromatids
Metaphasechromosome
5 µm
Chromosome terminology
Homologous chromosomes Are the two chromosomes composing a
pair One from each parent Have the same inherited characteristics
for each locus Autosomes are homologous
chromosomes not directly involved in determining gender
Sex chromosomes
Are distinct from each other in their characteristics
Are represented as X and Y Determine the gender of the individual,
XX being female, XY being male
A diploid cell
The number of chromosomes in a single set is represented by n.
A diploid cell has two sets (2n) of each of its chromosomes
In a human has 46 chromosomes (2n = 46)
In a cell in which DNA synthesis has occurred
All the chromosomes are duplicated and thus each consists of two identical sister chromatids
Figure 13.4
Key
Maternal set ofchromosomes (n = 3)
Paternal set ofchromosomes (n = 3)
2n = 6
Two sister chromatidsof one replicatedchromosome
Two nonsisterchromatids ina homologous pair
Pair of homologouschromosomes(one from each set)
Centromere
A haploid cell
• A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n)
• For humans, the haploid number is 23 (n = 23)• Each set of 23 consists of 22 autosomes and a
single sex chromosome• In an unfertilized egg (ovum), the sex
chromosome is X• In a sperm cell, the sex chromosome may be
either X or Y
Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity The ovaries and testes produce haploid
gametes by meiosis During fertilization
These gametes, sperm and ovum, fuse, forming a diploid zygote
The zygote Develops into an adult organism
Figure 13.5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
SpermCell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploidzygote(2n = 46)
Mitosis anddevelopment
Multicellular diploidadults (2n = 46)
The human life cycle
The Variety of Sexual Life Cycles
The three main types of sexual life cycles Differ in the timing of meiosis and
fertilization
Textbook does not have specific examples but the photographic atlas in lab does
In animals
Gametes
Figure 13.6 A
Diploidmulticellular
organism
Key
MEIOSIS FERTILIZATION
n
n
n
2n2nZygote
Haploid
Diploid
Mitosis
(a) Animals
Meiosis occurs during gamete formation
Gametes are the only haploid cells
MEIOSIS FERTILIZATION
nn
n
nn
2n2n
Haploid multicellularorganism (gametophyte)
Mitosis Mitosis
SporesGametes
Mitosis
Zygote
Diploidmulticellularorganism(sporophyte)
(b) Plants and some algaeFigure 13.6 B
Plants and some algae
Exhibit an alternation of generations
The life cycle includes both diploid and haploid multicellular stages
MEIOSIS FERTILIZATION
n
n
n
n
n
2n
Haploid multicellularorganism
Mitosis Mitosis
Gametes
Zygote(c) Most fungi and some protistsFigure 13.6 C
In most fungi and some protists
Meiosis produces haploid cells that give rise to a haploid multicellular adult organism
The haploid adult carries out mitosis, producing cells that will become gametes
Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis
However, only diploid cells can undergo meiosis
In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring
Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid
Meiosis Takes place in two sets of divisions,
meiosis I and meiosis II
Results in four daughter cells with half as many chromosomes
Stages of Meiosis
Meiosis I Reduces the number of chromosomes
from diploid to haploid Called reduction division because it
halves the numbers of chromosome sets per cell
Meiosis II Produces four haploid daughter cells
BioFlix: MeiosisBioFlix: Meiosis
Fig. 13-7-1
Interphase
Homologous pair of chromosomesin diploid parent cell
Chromosomesreplicate
Homologous pair of replicated chromosomes
Sisterchromatids Diploid cell with
replicated chromosomes
Fig. 13-7-2
Interphase
Homologous pair of chromosomesin diploid parent cell
Chromosomesreplicate
Homologous pair of replicated chromosomes
Sisterchromatids Diploid cell with
replicated chromosomes
Meiosis I
Homologouschromosomesseparate
1
Haploid cells withreplicated chromosomes
Fig. 13-7-3
Interphase
Homologous pair of chromosomesin diploid parent cell
Chromosomesreplicate
Homologous pair of replicated chromosomes
Sisterchromatids Diploid cell with
replicated chromosomes
Meiosis I
Homologouschromosomesseparate
1
Haploid cells withreplicated chromosomes
Meiosis II
2 Sister chromatidsseparate
Haploid cells with unreplicated chromosomes
Centrosomes(with centriole pairs)
Sisterchromatids
Chiasmata
Spindle
Tetrad
Nuclearenvelope
Chromatin
Centromere(with kinetochore)
Microtubuleattached tokinetochore
Tertads line up
Metaphaseplate
Homologouschromosomesseparate
Sister chromatidsremain attached
Pairs of homologouschromosomes split up
Chromosomes duplicateHomologous chromosomes
(red and blue) pair and exchangesegments; 2n = 6 in this example
INTERPHASE MEIOSIS I: Separates homologous chromosomes
PROPHASE I METAPHASE I ANAPHASE I
Interphase and meiosis I
Figure 13.8
TELOPHASE I ANDCYTOKINESIS
PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS
MEIOSIS II: Separates sister chromatids
Cleavagefurrow Sister chromatids
separate
Haploid daughter cellsforming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes
Two haploid cellsform; chromosomesare still doubleFigure 13.8
Telophase I, cytokinesis, and meiosis II
A Comparison of Mitosis and Meiosis
Meiosis and mitosis can be distinguished from mitosis by three events in Meiosis l1. Synapsis and crossing over
2. Tetrads on the metaphase plate
3. Separation of homologues
Fig. 13-9a
MITOSIS MEIOSIS
MEIOSIS I
Prophase I
Chiasma
Chromosomereplication
Homologouschromosomepair
Chromosomereplication
2n = 6
Parent cell
Prophase
Replicated chromosome
Metaphase Metaphase I
Anaphase ITelophase I
Haploid n = 3
Daughter cells ofmeiosis I
MEIOSIS II
Daughter cells of meiosis II
nnnn
2n2n
Daughter cellsof mitosis
AnaphaseTelophase
Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution
Reshuffling of genetic material in meiosis Produces genetic variation
In species that produce sexually The behavior of chromosomes during
meiosis and fertilization is responsible for most of the variation that arises each generation
Independent Assortment of Chromosomes
Homologous pairs of chromosomes Orient randomly at metaphase I of
meiosis In independent assortment
Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs
Origins of Genetic Variation Among Offspring
• Three mechanisms contribute to genetic variation: Independent assortment of chromosomes Crossing over Random fertilization
Independent assortment
Figure 13.10
Key
Maternal set ofchromosomesPaternal set ofchromosomes
Possibility 1
Two equally probable arrangements ofchromosomes at
metaphase I
Possibility 2
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
Human variation
• The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
• For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes
Crossing Over
Produces recombinant chromosomes that carry genes derived from two different parents
Figure 13.11
Prophase Iof meiosis
Nonsisterchromatids
Tetrad
Chiasma,site ofcrossingover
Metaphase I
Metaphase II
Daughtercells
Recombinantchromosomes
Random Fertilization
• Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)
• The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations
Evolutionary Significance of Genetic Variation Within Populations
Genetic variation Is the raw material for evolution by
natural selection Random variation with independent
assortment and crossing over will produce a zygote with any of about
64 trillion diploid combinations (in humans)
Evolution
Mutations Are the original source of genetic
variation Sexual reproduction
Produces new combinations of variant genes, adding more genetic diversity
Selection pressure on variation that is heritable causes natural selection leading to evolution